Although pyridine-free 1 is stable for several weeks at -45 OC,1 -tBupy is significantly less stable and decomposes to theamidoiron(III) complex (LMe*)Fe(NHAd) (tBupy) (3 tBupy)within a few hours at room temperature.37 In this reaction, theLMe ligand undergoes intramolecular C-C coupling to form anasymmetric dianionic ligand (designated LMe* here), presumablyby way of HAT (Scheme 2). A solution of 1 - tBupy also rapidlyreacts with 1,4-cyclohexadiene (CHD) to afford the amidoiron-(II) complex LMeFe(NHAd)(tBupy) (2*tBupy) and benzene(Scheme 3).37 Kinetic studies presented below are consistentwith the mechanisms in Schemes 2 and 3.Thermodynamics of Pyridine Coordination to 1. A neces-sary first step in the mechanistic inquiry is to quantify theequilibrium between 1 and 1 tBupy. The equilibrium constantsfor binding of tBupy to compound 1 were explored by evaluatingthe 1H NMR signals of mixtures of 1 and tBupy as a function of[tBupy]. Plotting the change in chemical shift versus [tBupy]oand fitting the data to a standard weak-binding equation41 gavevalues for Keq in the temperature range -51 to +40 OC (Table 1).The van't Hoff plot (Figure 1) gives values of AHeq =-7.0(2) kcal/mol and ASeq = -20.6(6) cal/mol K, whichare reasonable values for the proposed equilibrium: the exother-mic AHeq value is consistent with a new bond being formed, andthe negative ASeq value is consistent with combining two mol-ecules into one. At 298 K, these values correspond to AGeq =-0.9(3) kcal/mol, a value that indicates very weak coordination.Keq values for 4-phenylpyridine, 4-(dimethylamino)pyridine, and4-(trifluoromethyl)pyridine were also determined at 40 OC. Table 1shows that more electron-donating pyridines bind more strongly to1, as expected from their greater basicity.Kinetic Studies of the Intramolecular HAT Reaction. Theintramolecular hydrogen atom abstraction reaction (conversionof 1* tBupy to 3 tBupy, Scheme 2) was studied by 1H NMRspectroscopy in C6D6 at 40 OC with [1] = 29 mM. The progressof the reaction was followed by monitoring the disappearance of1 relative to a capillary integration standard.42 The concentrationof 1 decreased over time, and the relative integrations were fit to afirst-order exponential curve43 to obtain the observed pseudo-first-order rate constant kobs. A plot of kobs versus [tBupy] isshown in Figure 2. At low [tBupy], the observed rate has a nearlinear dependence on [tBupy], but the observed rate begins tosaturate at [tBupy] 0.2 M. The observed saturation kineticsindicate a rapid pre-equilibrium involving tBupy association priorto the rate determining step. The rapid equilibrium is consistentwith the dynamic averaging of signals in the 1H NMR spectrumof 1 and 1- tBupy.

0.0 0.20 0.40 0.60 0.80 1.0[tBupy] (M)Figure 2. Observed rate of intramolecular HAT as a function of tBupyconcentration. Data obtained in C6D6 at 40(1) OC with [Fe] = 29 mM.The dashed line represents the fit to eq 3.The intercept near the origin is consistent with the observationthat decomposition of 1 proceeds at a greatly decreased rate at40 oC without pyridine. When combined with the dependence ofkobs on [tBupy], this indicates that 1 . tBupy, and not 1, is the activespecies in the HAT reaction. These data imply the rate law in eq 2,where kintra is the first-order rate constant of the elementary HATstep.

d[ l'tBupy]dt

kintra [1 tBupy]

(2)

Since tBupy coordinates weakly to 1 at 40 OC (Keq = 2.5(1) M-1,see above), we can use the weak-binding approximation[1 tBupy]eq ' (Keq[1]o[tBupy]o)/(1 + Keq[tBupy]o) to obtainthe rate law in eq 3, which expresses the rate in terms of the known